Nullable $ref property (anyOf/oneOf with type: null) is silently omitted from the generated type #950

Summary

A property declared as a nullable reference — anyOf: [{$ref: …}, {type: "null"}] — is silently omitted from the generated Swift struct. No warning, no error, no diagnostic of any kind. The build succeeds and the property simply does not exist.

The same happens with the oneOf spelling. Sibling properties on the same schema generate normally, so the failure is per-property and easy to miss.

This is the standard shape FastAPI/Pydantic emits for an Optional[SomeModel] field, so it is not an exotic construct.

Reproduction

Minimal OpenAPI 3.1 document:

{
  "openapi": "3.1.0",
  "info": { "title": "Repro", "version": "1.0.0" },
  "paths": {
    "/thing": {
      "get": {
        "operationId": "getThing",
        "responses": {
          "200": {
            "description": "ok",
            "content": {
              "application/json": {
                "schema": { "$ref": "#/components/schemas/Thing" }
              }
            }
          }
        }
      }
    }
  },
  "components": {
    "schemas": {
      "Nested": {
        "type": "object",
        "properties": { "name": { "type": "string" } },
        "required": ["name"]
      },
      "Thing": {
        "type": "object",
        "properties": {
          "id": { "type": "integer" },
          "plain_nullable": { "type": ["string", "null"] },
          "ref_nullable_anyof": {
            "anyOf": [{ "$ref": "#/components/schemas/Nested" }, { "type": "null" }]
          },
          "ref_nullable_oneof": {
            "oneOf": [{ "$ref": "#/components/schemas/Nested" }, { "type": "null" }]
          },
          "ref_plain": { "$ref": "#/components/schemas/Nested" }
        },
        "required": ["id"]
      }
    }
  }
}

Config:

generate:
  - types
  - client
accessModifier: public

Expected

Thing has five properties, with the nullable references generated as optionals of Components.Schemas.Nested.

Actual

Thing has three. Both nullable-reference properties are gone:

public struct Thing: Codable, Hashable, Sendable {
    public var id: Swift.Int
    public var plain_nullable: Swift.String?
    public var ref_plain: Components.Schemas.Nested?
}

ref_nullable_anyof and ref_nullable_oneof are absent, and absent from CodingKeys too. plain_nullable (type-array nullability) and ref_plain (non-nullable $ref) both generate correctly — so the trigger is specifically reference + null.

Versions

Reproduced identically on 1.12.2 and 1.13.1, via the SPM build-tool plugin, Xcode 26, generating for an iOS target.

Workaround

Inlining the referenced schema in place of the $ref, with "type": ["object", "null"], generates the property correctly (as a nested …Payload type). That costs the shared named type at the embedding site, but it is the only shape we found that works. Rewriting anyOf → oneOf does not help.

Impact

The silence is the real problem. We shipped a hand-written workaround — an extra list fetch plus normalized-string matching — to reconstruct an object the API had been sending all along, because the generated type simply had no field for it. That went unnoticed for about two months. A diagnostic would have turned this into a five-minute fix.

Even if generating these is out of scope for now, emitting a warning for a property that is being dropped would prevent the silent-data-loss failure mode.